Method for forming a cored concrete slab

ABSTRACT

A method for forming cored concrete slab utilizing an elongated fixed mold for forming the opposite sides and bottom surface of the slab. A concrete depositing and core forming assembly moves longitudinally over the mold, the assembly including a plurality of elongated core forming mandrels and a hopper which deposits a first layer of concrete in the mold generally to the top of the mandrels. A length of metal mesh is provided having its free end attached at one end of the mold, the mesh being coiled on a reel mounted on the assembly which lays the mesh over the mandrels and the first layer, applying tension thereto, as the assembly moves toward the other end of the mold. Another hopper is mounted on the assembly which deposits a second layer of concrete over the first layer and mesh. The concrete has a slump of from 1 to 2 inches and the mesh is chosen so as to act as a concrete supporting material so as to allow removal of the mandrels at a rate faster than the concrete becomes self supporting whereby the mesh prevents collapsing of the cores. The mandrels may be rotated as they are withdrawn from the concrete and the method may also be utilized to form prestressed cored slabs.

United States Patent 1 Yost 1 1 METHOD FOR FORMING A CORED CONCRETE SLAB [75] Inventor: Robert L. Yost, Decatur, 1nd.

[731 Assignee: Joseph J. Dues, Dayton, Ohio [22] Filed: Sept. 14, 1971 [21] Appl. No: 180,442

Related U.S. Application Data [62] Division of Ser No. 771,152, Oct. 28, 1968, Pat. No.

[52] U.S. Cl. 264/72; 264/228; 264/308; 264/D1G, 57 [51] Int. Cl B28b 1/08 [58] Field of Search 264/33, 34, 228, 229, 256, 264/308, D10. 71, 72; 249/20, 21, 22; 425/63, 64, 111, 130, 427, 429, 134, 262

[56] References Cited UNITED STATES PATENTS 1,920,716 8/1933 Shafer 425/111 2,994,119 8/1961 Gagne 425/427 X 3,143,781 8/1964 Kalns 425/432 3,146,508 9/1964 Berliner 264/228 X 3,159,897 12/1964 Ellis 1 425/427 3,181,222 5/1965 Palmer 264/228 X 3,217,375 11/1965 Kinnard 264/256 X 3,382,304 5/1968 Nagy v 425/130 x 3,475,800 11/1969 Jones 425/111 Primary Examiner-Robert F. White Assistant Examiner-Thomas P. Pavelko {57] ABSTRACT A method for fomiing cored concrete slab utilizing an elongated fixed mold for forming the opposite sides and bottom surface of the slab. A concrete depositing and core forming assembly moves longitudinally over the mold, the assembly including a plurality of elongated core forming mandrels and a hopper which deposits a first layer of concrete in the mold generally to the top of the mandrels. A length of metal mesh is provided having its free end attached at one end of the mold, the mesh being coiled on a reel mounted on the assembly which lays the mesh over the mandrels and the first layer, applying tension thereto, as the assembly moves toward the other end of the mold. Another hopper is mounted on the assembly which deposits a second layer of concrete over the first layer and mesh. The concrete has a slump of from 1 to 2 inches and the mesh is chosen so as to act as a concrete supporting material so as to allow removal of the mandrels at a rate faster than the concrete becomes self supporting whereby the mesh prevents collapsing of the cores. The mandrels may be rotated as they are withdrawn from the concrete and the method may also be utilized to form prestressed cored slabs.

7 Claims, 8 Drawing Figures 1 METHOD FOR FORMING A CORED CONCRETE SLAB This is a Divisional application of application Ser. No. 77l.l52 filed Oct. 28. I968 now US. Pat. No. 3.647.308.

BACKGROUND OF THE INVENTION l. Field of the Invention This invention relates generally to apparatus and methods of forming reinforced concrete structural elements. and more particularly to an apparatus and method for forming a cored concrete slab from low slump concrete.

2. Description of the Prior Art Prestressed reinforced concrete beams. slabs. panels and other structural elements are extensively employed in the construction industry. Many of such elements are of the cored type. i.e. having cavities or cores therein for reducing the overall weight.

Such cored concrete structural elements are commonly formed by one of two different processes. The first process. employs a concrete mixture having no slump. This mixture is pressure formed into the desired shape. In one form of apparatus utilizing no slump concrete. an elongated stationary mold is employed which forms the sides and bottom surface of the element. An extruding device moves along the mold carrying a top plate which completes the mold cavity, and the relatively dry concrete mixture is forced into the cavity by rotating augers which also respectively form the cores in the product. In another apparatus utilizing no slump concrete an extruding assembly moves over a bed successively depositing three layers of relatively dry con crete mix. each layer being compacted by reciprocating tamping devices. The middle layer surrounds a plurality of reciprocating mandrels which form the cores, and the sides of the slab are formed by a slip form device which advances with the assembly. Both of the abovedescribed types of apparatus for performing the dry process possess certain disadvantages. One such disadvantage is that it has not been possible to incorporate inserts. such as electrical junction boxes. conduits and the like. into slabs formed in this manner.

In the second method of forming prestressed cored concrete structural elements. concrete having a low slump is poured in the mold. cured and removed therefrom. There is a preference in the industry for using concrete having a low slump rather than concrete having no slump. particularly where it is desired to include various types of inserts in the product. In one comparatively simple form of apparatus employing low slump concrete, a stationary mold is provided which forms the sides and bottom surface of the product. Elongated inflated mandrels are positioned in the mold and the relatively fluid or wet concrete is then poured into the mold covering the mandrels. After curing, the mandrels are deflated and removed leaving the cores in the product. In another type of apparatus employing low slump concrete, referred to as a slip form machine, an assembly is moved over a bed. The slip form machine forms the sides of the product; the bed forms the bottom surface of the slab. This assembly also carries a plurality of elongated mandrels or core tubes which form the cores in the product. the relatively wet concrete being poured over the mandrels into the mold cavity formed by the slip form machine and the stationary bed. The

speed of operation of the slip form machine is necessarily limited by the fact that the moving assembly carries a portion of the mold and thus can travel only as fast as the concrete cures so as to be self-supporting.

SUMMARY OF THE INVENTION It is desired to provide an apparatus and method for forming a prestressed. cored concrete slab employing low slump concrete which incorporates all of the advantages of such a process.

In accordance with the broader aspects of the invention. a longitudinally elongated mold is provided for forming the opposite sides and bottom surface of the slab. Means are provided for progressively laying a layer of cementitious material in the mold and including means disposed over the mold for receiving cementitious material in relatively fluid form and depositing the same in the mold. and means are provided for relatively longitudinally moving the depositing means with respect to the mold in the direction of elongation thereof. A plurality of elongated mandrels are provided disposed in the mold in spaced. parallel relationship and extending in the direction of relative movement for forming the longitudinally extending cores in the slab. the mandrels having spaced opposite ends and having portions intermediate their ends disposed under the depositing means. Means are provided for attaching the leading ends of the mandrels which face in the direction of relative movement to the laying means so that the mandrels are relatively moved with the depositing means. In the preferred embodiment, the layer of cementitious material is deposited to a level generally even with the tops of the mandrels and an elongated length of mesh reinforcing material is provided having its free end attached to the mold, the laying means laying the mesh. under tension. over the previously deposited layer, the laying means including means for depositing a second layer of cementitious material in relatively fluid form over the first layer and mesh.

It is accordingly an object of the invention to provide improved apparatus for forming a cored concrete product.

Another object of the invention is to provide an improved apparatus employing concrete having a low slump, in contrast to no slump, for forming a prestressed, cored concrete product.

A further object of the invention is to provide an improved method for forming a cored concrete slab from concrete having a low slump in contrast to no slump.

The above-mentioned and other features and objects of this invention and the manner of attaining them will become more apparent and the invention itself will be best understood by reference to the following description of an embodiment of the invention taken in conjunction with the accompanying drawings, wherein:

BRIEF DESCRIPTION OF THE DRAWINGS FIG. I is a fragmentary view in perspective, partly broken away. illustrating the apparatus of the invention;

FIG. 2 is a top view of the apparatus illustrated in FIG. 1;

FIG. 3 is a side view of the apparatus illustrated in FIG. I, partly broken away;

FIG. 4 is a fragmentary cross-sectional view. partly schematic, taken generally along the line 4-4 of FIG. 2;

FIG. 5 is a fragmentary cross-sectional view, taken generally along the line 5-5 of FIG. 2;

FIG. 6 is a generally schematic diagram showing the propulsion system of the apparatus;

FIG. 7 is a fragmentary view in perspective. partly in crosssection, useful in explaining the method of the invention, and further illustrating certain features of the apparatus; and

FIG. 8 is a fragmentary cross-sectional view taken along the line 8-8 of FIG. 7.

DESCRIPTION OF THE PREFERRED EMBODIMENT The Apparatus Referring now to the drawings, the apparatus of the invention, generally indicated at 10, comprises a mold assembly 12 for forming the parallel opposite sides and bottom surface of slab l4, and an assembly 16 which travels on the mold depositing cementitious material therein, forming the core openings in the slab, and finishing the top surface as its forward movement in the direction shown by the arrow I8 progresses.

Mold I2, only a short section of which is shown, but which may be several hundred feet in length, comprises a bed 20 supported upon a supporting surface 22, such as the ground, by a plurality of longitudinally spacedapart support elements 24. Bed 20 may have conduits 26 formed therein for circulating a heating medium, such as heated oil, in order to accelerate the curing of the slab in the mold. A pair of side mold elements 28, are provided on the opposite sides of bed 20 and extend longitudinally the entire length thereof. Side mold elements 28 and 30 have rails 32 and 34 formed on their upper extremities for supporting the assembly 16 for longitudinal movement with respect to the mold 12. Side mold elements 28 and 30 are supported upon the supporting surface 22 by a plurality of spaced-apart, vertically adjustable supports 36, and are adapted to be vertically raised and lowered between upper and lower positions by a plurality of conventional hydraulic jacks 38. In their upper positions, side mold elements 28 and 30 extend upwardly above the upper flat surface of bed 20. The inner surfaces 42 and 44 of side mold elements 28 and 30 thus define a mold cavity having an open top. The elements 28, 30 with upper surface 40 of base 20 form the opposite parallel sides and bottom surface of the slab 14. In the lower positions of side mold elements 28 and 30, rails 32 and 34 are flush with the upper surface 40 of base 20, or lower, thereby to assist in the removal of the slab 14 from the mold cavity and to permit transverse sawing of the elongated slab 14 in the mold into shorter elements of desired length by a conventional concrete sawing apparatus (not shown).

The assembly 16 comprises a supporting frame 46 supported on the rails 32 and 34 of the side mold elements 28 and 30 for longitudinal movement thereon by means of a forward pair of wheels 48 and 50 and a rear pair of wheels 52 and 54. The forward pair of wheels 48 and 50 are respectively mounted on an axle 56 and the rear pair of wheels 52 and 54 are likewise mounted on an axle 58, axles 56 and 58 being mounted on frame 46, as shown.

The assembly 16 is moved longitudinally over the mold 12 in the direction shown by the arrow 18. The assembly 16 is guided to move on the rails 32 and 34 by the guides 47. The guides 47 are located at each cor- 4 ner of the frame 46 and engage the facing surfaces 42 and 44 of elements 28 and 30.

The wheels 48, 50, 52 and 54 are driven by the propulsion system shown in FIG. 6. A suitable electric motor 60 is mounted on frame 46 and drives a conventional hydraulic pump 62 through a conventional belt drive 64. Pump 62 receives hydraulic fluid from hydraulic tank 66 through supply line 68. Hydraulic pump 62 independently drives the forward pair of wheels 48 and 50, and the rear pair of wheels 52 and 54, respectively, through conventional hydraulic motors 70 and 72. Pump 62 is connected to the forward hydraulic motor 70 by pressure line 74, hydraulic motor 70 having a discharge line 76 which is returned to the tank 66. Pump 62 likewise drives the hydraulic motor 72 through pressure line 78, motor 72 having discharge line 80 which is returned to the tank 66. The forward hydraulic motor 70 drives a suitable gear box 82 which, in turn, drives the forward wheels 48 and 50 through sprockets 84 and 86, drive chain 88, jack shaft 90, sprockets 92 and 94, and drive chains 96 and 98 respectively driving the forward wheels 48 and 50 through sprockets 100, 102 thereon. The rear hydraulic motor 72 similarly drives gear box 104 which drives the rear wheels 52 and 54 through sprockets 106 and 108, drive chain 110, jack shaft 112, sprockets 114 and 116, drive chain 118, jack shaft 120, and sprockets 122, 124 which respectively drive rear wheels 52, 54 through drive chains 126 and 128, driving sprockets I30, I32 thereon.

Motor 60 may be energized by suitable flexible ca bles, or more preferably by a conventional third rail and sliding contact system (not shown). Suitable motor controls 134 are provided for controlling the speed of motor 60 and thus the linear speed at which the assembly 16 moves in the direction 18. Suitable catwalks 136 are supported on frame 46 for the operator and provide access to the motor controls 134.

An elongated hopper 138 is mounted on frame 46 and extends transversely thereacross over the mold 12 for depositing cementitious material in the mold cavity to form a first layer 140 therein as the assembly 16 moves forwardly in the direction 18. Hopper 138 has an open upper end 142 for receiving the cementitious material, and forward and rear side walls 144 and 146 having converging lower extremities 148 and 150 which define an elongated, transversely extending discharge opening 152 for depositing the cementitious material in the mold over the top surface 40 of base 20. An elongated force feeding and distributing auger 154 is positioned in hopper 138 adjacent opening I52. Auger 154 is selectively driven in opposite directions by means of a conventional hydraulic motor 156 which drives gear box 158 which, in turn, drives auger 154 through drive chain 160. Hopper 138 may be continuously filled with cementitious material by means of a conventional readymix truck (not shown) which moves along the side of the mold 12 as the assembly 16 moves forwardly in direction 18 or by like means.

A plurality of elongated core-forming mandrels 162 are provided in the mold cavity and extending in spaced, parallel relationship in the direction of movement. Mandrels 162 are in the form of hollow metal tubes closed at their opposite ends I64, I66 and have elongated rod portions and "I secured to the mandrels 162 at their opposite ends, rod portions 170 and 171 respectively project beyond opposite ends 164 and 166. The portions 170 which project forwardly from the forward or leading ends 164 of mandrels 162, which face toward the direction of movement 18, are connected to frame 46 for rotational movement by downwardly extending brackets 172. It will thus be seen that portions of mandrels 162 as adjacent their leading ends 164 extend under the discharge opening 152 of hopper 138, the rear or trailing ends 166 of mandrels 162 extending substantially rearwardly of the rear end 174 of frame 46. In a specific embodiment of the apparatus of the invention, mandrels 162 were each feet long and extended 20 feet beyond rear end 174 of frame 46.

It will thus be seen that the mandrels 162 are mounted for rotational movement about their longitudinal axes which extend between their leading and trailing ends 164, 166, rod portions 170, 171 being coaxial therewith, and leading ends 164 of mandrels 162 being pulled by brackets 172 as the assembly 16 moves forwardly in direction 18, the trailing ends 166 being unsupported by the frame 46 and floating in the layer 140 of cementitious material so as to form the core openings therein. In order to keep the mandrels 162 submerged in the cementitious material, of the layer 140 and to keep the mandrels162 generally parallel to the surface 40, mandrels 162 are wholly or partially filled with liquid 176, preferably a mixture of water and alcohol, so that the weight of mandrels 162 is such that the trailing ends 166 will not float upwardly or sink from the afore-mentioned desired position in the layer 140. In a preferred embodiment, conventional electrically energized immersion heaters 178 are provided within mandrels 162 for heating the same thereby to accelerate curing of the cementitious material.

Mandrels 162 have a limited rotary oscillatory trowcling motion imparted thereto about their axes by the arrangement now to be described. Referring specifically to FIG. 4 in the illustrated embodiment, one group 180 of the mandrels 162 respectively have ends 182 of lever members 184 secured to projecting portions 170 of rods 168. The upper ends 166 of lever members 184 are respectively pivotally connected together by members 188. A conventional hydraulic cylinder 190 is provided having its rear end pivotally connected to a side of frame 46, as at 192, and having its piston rod 194 connected to members 188, as at 196. It will thus be seen that protractile movement of piston rod 194 of cylinder 190 will result in movement of members 198 and levers 184 to the position shown in dashed lines in FIG. 4, whereas retractile movement of piston rod 194 will move members 188 and levers 184 to the position shown in solid lines. Thus, reciprocatory motion of piston rod 194 results in reciprocatory motion of members 188, as shown by the arrows 198, in turn resulting in rotary, oscillatory troweling motion of mandrels 162, as shown by the arrows 200. Suitable limit switches, schematically shown at 202 and 204, respectively sense the two extremities of the reciprocatory movement of members 188 and are coupled to a suitable solenoid reversing valve 206 for controlling the reciprocatory action of hydraulic cylinder 190. In the illustrated embodiment of the apparatus which incorporates twelve mandrels 162, the mandrels are divided into two groups 180 each having the rotary, oscillatory troweling motion imparted thereto by a cylinder 190, as best seen in FIG. 2.

In the preferred embodiment of the assembly 16, mandrels 162 rotate about one-fourth revolution, and hopper 138 is disposed and auger 154 is driven so as to deposit layer 140 of cementitious material to a level so that its upper surface 208 is substantially even with the tops of the mandrels 162, as best seen in FIG. 8. In this connection, it will be observed that the edge of sidewall 146 of hopper 138 will provide a bulldozing action to assist in depositing the layer 140 to the level 208.

A plurality of vibrating devices 210 are disposed between mandrels 162 and adjacent to side mold ele ments 28, 30, closely adjacent rear edge 150 of discharge opening 152 of hopper 138, vibrating devices 210 being further disposed adjacent the tops of man drels 162, as best seen in FIGS. 3 and 5. Vibrating devices 210 are individually driven by variable speed electric drive motors 212 mounted on frame element 214 forwardly of front wall 144 of hopper 138, each motor 212 being connected to and supporting its respective vibrating device 210 by conduit 216 enclosing a conventional flexible drive shaft. The vibrating devices 210 function to continuously pack the cementi tious material between the mandrels 162 and side ele ments 28, 30 as it is deposited by the auger 154 from the hopper 138. The speeds of the drive motors 212 and thus of the vibrating devices 210 are individually selectively controlled by speed controls 218 mounted on control panel 220.

In accordance with the invention, an elongated length of metal mesh material 222 is provided having a width slightly less than the width of the mold 12 be tween the side mold elements 28,30. As will be hereinafter more fully described, free end 224 of mesh 222 is initially secured to a member 226 which is mounted ad jacent the rear or starting end 228 of mold 12. Mesh 222 extends forwardly over mandrels 162 and is accommodated on and payed-out by a reel 230 rotatably supported by members 232 extending upwardly from frame 46, reel 230 being disposed rearwardly of the hopper 138. Mesh 222 is trained around guide rollers 234 which are respectively disposed in alignment with the spaces between the mandrels 162, rollers 234 pressing the mesh 222 downwardly against the tops of mandrels 162 and into the top surface 208 of cementitious material layer 140 between the mandrels, as best seen in FIG. 8. Mesh 222 is maintained in tension as the assembly 16 moves forwardly in direction 18 by means of a suitable brake device 236 acting upon reel 230. It will thus be seen that as the forward movement of assembly 16 progresses in direction 18, mesh 222 is laid, under tension, over cementitious material layer 140 and mandrels 162, being pressed into the top surface 208 of layer 140 by roller 234.

A second elongated hopper 236 is mounted on extension portion 238 of frame 46 which extends rearwardly from the rear end 174, hopper 236 extending transversely across mold 12 over mandrels 162, cementitious material 140 and mesh 222, and being similar in construction to hopper 138. Hopper 236 has an upper open end 240, a lower discharge opening 242, and a force feeding and distributing auger 244 therein adjacent the discharge opening 242. Hopper 236 receives cementitious material and in conjunction with auger 244 deposits a second layer 246 over mandrels 162, the first layer 140 and mesh 222, as best seen in FIG. 8. Auger 244 is selectively driven in opposite directions by another hydraulic motor 248 which drives gear l)\ 250. which. in turn. drives auger 244 through a drive cha n 252.

Hopper 236 is preferably continuously filled by means of an elevating auger 254 and an open trough 256 which is inclined upwardly from adjacent the forward hopper E38 toward the rear hopper 236. as best seen in 2G. 3. Elevating auger 254 is driven by a conventional electric motor 253 through a belt 261). Thus. by employing two separate chutes rom a readymix trucs. :r two separate like de ices. one for depositing the cementitious material into the hopper i258, and the other for depositing cementitious material in the trough Sti for elevation by auger 254 and depositing in the rear hopper 2%. both hoppers 133 and 23!) can be supplied with cementitious material.

The second layer 246 of cementitious material is compacted and its top surtace 262 is finished by a vibratory screed 26- supported on frame extension por tion 258. v'arious inserts. hardware or the like (not shownl may be placed in slab 1- after the mesh has been laid on top of the :nandrels before the mandrels have been removed from the slab portion in which the inser s are desirably placed. Adjacent to side elements 2 8. 39 and around such inserts. surface 262 must be fin shei by hand.

Fads E66 ol' mandrels Hi2 also may be secured to brackets 172 by means of projecting portions 17% of rods I68, both of the projecting portions Wt. and 17 being adapted to be removably rotatably mounted in brackets 1 2 and rcmovably connected to ends 182 olle ers 18-4. With this arrangement, when the apparatus 1? has advanced to the far end of mold 12, remote from the starting end 228, and slab has been completed and the resulting elongated slab has been sawed into shorter sections and the sections removed projecting ends 170 may be disconnected from brackets E72 and 'eve s I34, the assembly to can be turned encl-for-cnd. and the projecting ends l7! can be connected to bracl ets W2 and ends 182 of levers I34, thereby making cnds iti of manurels $62 the leading ends. The assembly 16 may then be advanced back to the former start ing end of the mold to form another slab, thus e iminating the necessity for conveying the entire assembly back to the starting end of the mold prior to forming another slab.

TH E \l ETHO D l rior to commencing motion of the assembly 16 o the rails 32 and 34 of the side mold elements 28 and 3t), transversely extending, longitudinally spaced-apart reinforcing strands 266 are placed in the mold cavity, and longitudinally extending. transversely spaced-apart prestressing strands .168 are placed thereovcr and extending the length of the mold. Strands 268 have their ends 270 secured to member 226 and their other ends {not shown) suitably tensioned, as by the use ofappropriate hydraulic tensioningjacks. Transverse and longitudinally extending reinforcing strands 266 and 268 are he d in assembled relation and spaced from the top surface ll of base by conventional spacers 272. Other hardware such as junction boxes Z74, window frames. conduits and the like may a so be positioned within the told cavity if desired. Assembly to is positioned on rails 37. and 34 with trailing ends lot) of mandrels Hi2 adjacent mold end 228.. and end 224 ofrnesh 222 is secured to member 226. If desired. cemcndtious material may be placed by hand in the mold rearvvardly from the front hopper 138 to the level of the tops of the man drels I62. and to the desired level 262 rearwardly of the rear hopper 236. Relatively fluid. wet cementitious material. for example concrete having a low slump. is then deposited from a rcadymix truck or like device in the hopper U8 and trough 256 and forward movement of the assembly it in the direction shown by the arrow 18 is commenced. As the forward movement progresses. the bottom layer 140 of cementitious material is deposited by trough 13-8 and auger 154 to a level substantially even with the tops of the mandrels 162, that layer is tamped between the mandrels and adjacent side elements 28. 30 by vibratory devices 216, the mesh 222 s payed-out from the reel 230, under tension. and is pressed onto the tops of the mandrels 162 and into the top surface 208 oflayer 140 by rollers 234, the second layer 246 is deposited over the mandrels 162. mesh 222 and the bottom layer 140 by hopper 23b and the auger 244, and that layer is compacted and its top surface 262 finished by the vibratory screed 264.

it. will be observed that the floating mandrets :2 have their leading cnds H34 pulled forwardly in the direction 18 by the forward motion of the assembly 16 thereby forming the core openings in the resulting slab. The mesh 222 serves not only as reinforcing material in the finished slab, but further. in combination with the cementitious layer 149, supports the top layer 246 of wet cementitious material so that the core openings formed by the advancing mandrels 162 do not collapse. it will be observed that the rotary oscillatory motion imparted to the mandrels 162 provides a troweling action for the interior surfaces of the core openings formed by the mandrels H62 as the mandrels advance in the direction 13.

There are a number of factors which must be corre lated in order to form a satisfactory slab. These include the linear rate of speed of the assembly 16 in direction 28. the formulation of the cemcntitious material used. including in one specific embodiment the cement aggregate-sand ration, the moisture content and curing time of the concrete mixture, the consistency or slump of the mixture, which is applied in wet or fluid form. the aggregate size and weight, and the size of the mesh 222 which must support the upper cementitious layer 246 in a wet condition. More particularly. the linear rate of speed, curing time. consistency and mesh size must be correlated so that the core openings formed by the mandrels 162, as they advance forwardly in direction 18, do not collapse. and the linear rate of speed, moisture content and curing time must further be correlated so that the requisite smooth finish is formed on the top surface 262 by the vibratory screed 264.

in a specific embodiment of the apparatus and method for forming slabs 8 inches thick and 8 feet wide, 12 mandrels respectively 30 feet long and having a 6 inch outside diameter were employed. A concrete mix having a slump from about l inch to about 2 inches is employed, preferably using a formula comprising from about 1000 lbs. to about 1300 lbs. of No. 9 stone; from about i700 lbs. to about 2000 lbs. of No. 14 sand, from about 650 lbs. to about 760 lbs. of high early cement: from about l8 to about ounces ofa water dis parsing agent (ICON. as sold by Master Builders); and from about to about gallons of water (correcting for moisture in gravel and sand, humidity, etc). Using a formula within this range. the assembly l6 can be moved at a linear forward speed within the range from about 1 foot per minute to about 6 feet per minute.

Particularly satisfactory results have been provided with a linear forward speed of about 2 feet per minute and a formula comprising about 1052 lbs. of No. 9 stone, 1933 lbs. of No. 14 sand; 752 lbs. of high early cement, 23 ounces of the above-identified water dispersing agent, and 40 gallons of water.

After formation of the slab 14 in the mold, curing may be further accelerated by the use of cover members 276 removably positioned on the top rails 32 and 34 of the side mold elements 28 and 30, and extending over the slab l4 and enclosing the mold cavity, cover members 276 having suitable electrical heating elements 278 therein.

What is claimed is:

l. The method of making a cored cementitious slab comprising the steps of providing an elongated mold having a bottom and upstanding sides for forming the bottom and side surfaces of a slab, placing a plurality of elongated mandrels in said mold adjacent to one end thereof and in a spaced and parallel relationship with respect to each other and said mold sides so as to extend longitudinally of said mold, and sequentially performing the following steps, depositing a first layer of cementitious material in relatively fluid form having a slump from about 1 inch to about 2 inches in said mold to a level generally even with the tops of said mandrels, laying elongated cementitious material supporting material in said mold over said mandrels and said first layer, depositing a second layer of cementitious material in relatively fluid form having a slump from about 1 inch to about 2 inches in said mold over said first layer and said cementitious material supporting material while continuously imparting troweling motion to each of said mandrels, moving said mandrels from adjacent to said one end of said mold toward the other end of said mold to remove the mandrels from the slab at a rate in excess of about 1 foot per minute and faster than said cementitious material becomes selfsupporting, and leaving said mold bottom and sides and said cementitious material supporting material behind said mandrels to support said cementitious material until self-supporting.

2. The method of claim 1 comprising the further step of applying tension to said cementitious material supporting material during said laying thereof.

3. The method of claim 1 wherein said cementitious material is a mixture of aggregate. sand. cement and water, and a water dispersing agent. said cementitious material having a low slump, said slump is from about 1 inch to about 2 inches, said aggregate comprises from about 1000 lbs. to about I300 lbs, of No. 9 stone, said sand comprises from about i700 lbs. to about 2000 lbs. of No. l4 sand, said cement and water comprises from about 650 lbs. to about 760 lbs. of high early cement and from about 30 gallons to about 40 gallons of water.

4. The method of claim I wherein said laying step and said depositing steps are performed at positions which are fixed with respect to said mandrels and move with said mandrels toward said other end as said slab is formed, each of said positions being between the opposite ends of said mandrels.

5. The method of claim 4 wherein said first layer depositing step is performed adjacent to said leading ends of said mandrels, said position at which said second layer depositing step is performed being spaced and remote from said position of said first layer depositing step, said laying step being performed at a position spaced from and between said depositing steps. both said depositing steps and said laying step being per formed simultaneously.

6. The method of claim 1 wherein each of said mandrels has a longitudinal axis between its ends, and said imparting step includes the step of imparting a limited rotary oscillatory motion to said mandrels about their respective axes.

7. The method of claim I further comprising the step of packing said first layer between each of said mandrels. 

1. THE METHOD OF MAKING A CORED CEMENTITOUS SLAB COMPRISING THE STEPS OF PROVIDING AN ELONGATED MOLD HAVING A BOTTOM AND UPSTANDING SIDES FOR FORMING THE BOTTOM AND SIDE SURFACES OF A SLAB, PLACING A PLURALITY OF ELONGATED MANDRELS IN SAID MOLD ADJACENT TO ONE END THEREOF AND IN A SPACED AND PARALLEL RELATIONSHIP WITH RESPECT TO EACH OTHER AND SAID MOLD SIDES SO AS TO EXTEND LONGITUDINALLY OF SAID MOLD, AND SEQUENTIALLY PERFORMING THE FOLLOWING STEPS, DEPOSITING A FIRST LAYER .OF CEMENTITIOUS MATERIAL IN RELATIVELY FLUID FROM HAVING A SLUMP FROM ABOUT 1 INCH TO ABOUT 2 INCHES IN SAID MOLD TO A LEVEL GENERALLY EVEN WITH THE TOPS OF SAID MANDRELS, LAYING ELONGATED CEMENTITIOUS MATERIAL SUPPORTING MATERIAL IN SAID MOLD OVER SAID MANDRELS AND SAID FIRST LAYER, DEPOSITING A SECOND LAYER OF CEMENTITIOUS MATERIAL IN RELATIVELY FLUID FROM HAVING A SLUMP FROM ABOUT 1 INCH TO ABOUT 2 INCHES IN SAID MOLD OVER SAID FIRST LAYER AND SAID CEMENTITIOUS MATERIAL SUPPORTING MATERIAL WHILE CONTINUOUSLY IMPARTING TROWELING MOTION TO EACH OF SAID MANDRELS, MOVING SAID MANDRELS FROM ADJACENT TO SAID ONE END OF SAID MOLD TOWARD THE OTHER END OF SAID MOLD TO REMOVE THE MANDRELS FROM THE SLAB AT A RATE IN EXCESS OF ABOUT 1 FOOT PER MINUTE AND FASTER THAN SAID CEMENTITIOUS MATERIAL BECOMES
 2. The method of claim 1 comprising the further step of applying tension to said cementitious material supporting material during said laying thereof.
 3. The method of claim 1 wherein said cementitious material is a mixture of aggregate, sand, cement and water, and a water dispersing agent, said cementitious material having a low slump, said slump is from about 1 inch to about 2 inches, said aggregate comprises from about 1000 lbs. to about 1300 lbs. of No. 9 stone, said sand comprises from about 1700 lbs. to about 2000 lbs. of No. 14 sand, said cement and water comprises from about 650 lbs. to about 760 lbs. of high early cement and from about 30 gallons to about 40 gallons of water.
 4. The method of claim 1 wherein said laying step and said depositing steps are performed at positions which are fixed with respect to said mandrels and move with said mandrels toward said other end as said slab is formed, each of said positions being between the opposite ends of said mandrels.
 5. The method of claim 4 wherein said first layer depositing step is performed adjacent to said leading ends of said mandrels, said position at which said second layer depositing step is performed being spaced and remote from said position of said first layer depositing step, said laying step being performed at a position spaced from and between said depositing steps, both said depositing steps and said laying step being performed simultaneously.
 6. The method of claim 1 wherein each of said mandrels has a longitudinal axis between its ends, and said imparting step includes the step of imparting a limited rotary oscillatory motion to said mandrels about their respective axes.
 7. The method of claim 1 further comprising the step of packing said first layer between each of said mandrels. 